Multilayer heat shrinkable film

ABSTRACT

A multilayer heat shrinkable film having high heat shrink, high burn-through resistance and a broad impulse sealing range including an outer and a heat sealable inner layer each of a thermoplastic polymer or copolymer, wherein said outer layer has a melting point at least 20° C. higher than said inner layer.

FIELD OF THE INVENTION

This invention relates to multilayer heat shrinkable film.

This invention has particular application to such films for use inpackaging or foods such as meat, poultry and the like. However, it isenvisaged that films in accordance with the present invention may finduse in other packaging applications such as packaging for curableputties and sealants.

BACKGROUND OF THE INVENTION

Primal meat cuts are generally smaller than a side of beef, but largerthan the ultimate cut sold to the retail consumer. Primal cuts areprepared at the slaughter house and shipped to retail meat stores orrestaurants where they are butchered into smaller cuts of meat commonlyreferred to as sub-primal meat cuts. Sub-primal cuts, however, may alsobe prepared at the slaughterhouse.

Primal and sub-primal cuts are packaged to minimize air (i.e. oxygen)exposure and prevent meat spoilage and discoloration during shipping andhandling. One common way to package primal and sub-primal cuts andprotect them from the adverse effects of air is to shrink package thecuts in a film having good oxygen barrier properties. For example, amultilayer film having a barrier layer containing polyvinylidenechloride (PVDC) is a well-known packaging material. PVDC is known tohave excellent oxygen barrier properties. The other layers of themultilayer film function to protect the PVDC layer and provide therequisite low temperature and abrasion resistance properties, which thePVDC layer lacks. There are other types of biaxially oriented heatshrinkable multilayer films, which do not require oxygen barrierproperties, for example films to be used for shrink wrapping poultry.

Shrinkage properties may be produced in a film by biaxially stretchingthe film in the machine and transverse direction. The resulting filmwill heat shrink within a specified range of percentages such as fromabout 20 to about 50 percent at about 90° C.

Multilayer heat shrinkable film may also be formed into packaging bags.Bags are generally made by transversely sealing a tubular stock ofmultilayer film and cutting or splitting the tube, or by superimposingflat sheets of multilayer film and sealing on three sides, or by endfolding flat sheets and sealing the two sides. A common type of sealused in manufacturing bags is a hot bar seal. The adjacent thermoplasticlayers, referred to as the inner layers, are sealed together byapplication of heat and pressure across the area to be sealed, usingcontact with opposing bars of which at least one is heated to cause theadjacent layers to fusion bond. Bags manufactured from a tube stockgenerally require one hot bar seal transverse to the tube. This seal isalso referred to as a bottom seal. Once the bottom seal is applied, thetube stock is transversely cut to form the mouth of the bag. Thestrength of seals may be measured by determining the time for a seal tofail when under certain conditions. For example, the seal is immersed inhot water at 95° C. and the hot water seal strength (“HWSS”) may bemeasured by a test such as that described as the “restrainedshrinkage-seal strength test” in Funderburk et al U.S. Pat. No3,900,635.

Once meat or poultry is inserted into the bag, the package is evacuatedand the bag mouth must be sealed. At one time, the standard method forsealing a bag was to fasten a clip around the mouth of the bag. Today,impulse heat sealing techniques are employed to seal the bag mouth. Ingeneral, a bag mouth is impulse sealed by application of heat andpressure using opposing bars. At least one of the bars has a coveredwire or ribbon through which electric current is passed for a very brieftime period (hence the name “impulse”) to cause the adjacent film layersto fusion bond. Following the impulse of heat the bars are cooled whilecontinuing to hold the bag inner surfaces together to achieve adequateseal strength.

One problem with impulse heat sealing is that the film in the seal areaoften becomes extruded during sealing. This elongation of the productresults in thinning of the film and in an extreme situation severing ofthe thinned film. The latter is known as burn-through. One solution tothis “burn-through” problem is to irradiate the film prior tomanufacture of the bag.

Irradiation of a multilayer film causes the various irradiated layers inthe film to cross-link. Under controlled conditions, cross-linking byirradiation not only provides a higher temperature impulse heat sealingrange, but also enhances the puncture resistance of the film. Undercontrolled conditions, polymeric multilayer films can be irradiated toproduce a cross-linked product having greater puncture resistancecharacteristics and other enhancements. However, exposure to irradiationalso causes a reduction of heat sealability in other layers atconventional temperatures, pressures and times using typical heatsealing equipment thereby resulting in poor bottom and edge seals inbags made from the film.

The equipment to irradiate the film is highly expensive, is costly tooperate, increases the product cost, and usually requires an added stepin the production operation.

Unfortunately, not all cross-linked thermoplastic films are easy tomelt, making it difficult for food packagers to achieve strong seals,particularly by impulse sealing the bag mouths after filling with meator poultry. All of the bag seals (including both the sealed bottom asfor example made by the bag manufacturers with a hot bar and theimpulse-sealed bag mouth by the food processor) must maintain theirintegrity when the food-containing package is immersed in hot water toshrink the film against the packaged food.

Prior art films typically used have the composition of the inner andouter layers with the same melting point and essentially the samepolymers. It was found that if the inner and outer layers are not ofsimilar composition the secondary bubble may not be stable during thebiorientation step and the film would tend to curl inward or outwardbecause the shrink of the inner and outer layers is not balanced. Inother words, it a high shrinkage resin in the inner layer is combinedwith a low shrink outer layer or vice versa the resultant film will curlor roll up and not be able to be fabricated into bags.

Generally higher melting point resins do not have the same shrinkage asthe lower melting point resins, hence the practice to use the sameresins in the inner and outer layers and enhance the sealability of thefilm by cross-linking.

In spite of this generalization, there are films, such as Krehalon ML 40and Vector 6 respectively made by Krehalon of Netherlands and Gunze ofJapan, which include an outer layer having a higher melting point thanthe heat sealing inner layer. Krehalon ML 40 has a polyester outerlayer, a polyethylene inner layer and core layers of polyamide andethylene vinyl alcohol. Vector 6 has a polyamide outer layer and apolyethylene inner layer. However, polyesters and polyamides are veryexpensive and can limit the shrink of the film. This is seen in thecases of Krehalon ML 40 and Vector 6 which respectively have film shrinkof 29/33% and 26/28% in the machine direction/ transverse direction(MD/TD).

SUMMARY OF THE INVENTION

This invention in one aspect resides broadly in a multilayer heatshrinkable film including an outer layer and a heat sealable innerlayer, wherein said outer layer has a melting point at least 20° C.higher than said inner layer and said layers have substantially balancedshrinkage, and wherein said film shrinks at least 35% in at least one ofmachine and transverse directions by measuring unrestrained shrink ofthe stretched film at 90° C. for five seconds, or equivalent shrinkagethereof.

More suitably, the film shrinks at least 40% in at least one of machineand transverse directions by measuring unrestrained shrink of thestretched film at 90° C. for five seconds, or equivalent shrinkagethereof.

By “substantially balanced shrinkage” it is meant that the inner andouter layers are compatible in shrink such that the composite structureof the film does not curl either inward or outward. It is to beunderstood that the shrinkage of each separate inner and outer removedfrom the composite structure of the film is not to be considered whendetermining when a film has substantially balance shrinkage.

The term “polymer”, as used herein, refers to the product of apolymerisation reaction, and is inclusive of homopolymers, copolymers,terpolymers etc.

Preferably, the outer layer is selected from a thermoplastic polymerblend. In the past, the composition of the outer layer was predeterminedby the selection of heat sealable inner layer to prevent curl duringbiorientation, but leading to the problems of burn-through and the needfor irradiation. It has been found that through the use of a blend ofthermoplastic polymers, that the temperature resistance of the outerlayer may be increased in comparison to the inner layer to preventburn-through while maintaining compatible shrink.

It is to be understood that the melting point of the blend is calculatedby averaging the melting points of the thermoplastic polymers in theblend taking into account the percentage of each polymer in the blend.The percentage of each polymer in the blend is multiplied by its meltingpoint and the sum of these values is the melting point of the blend. Forexample if polymer A has a melting point of 100° C. and polymer B has amelting point of 200° C. a 50% blend of each has a melting point of 150°C. If a blend comprises 40% polymer A and 60% polymer B the blend has amelting point of 160° C.

Preferably, the outer layer is a blend of polypropylene (PP) andpolybutylene (PB) and either a polyethylene (PE) or ethylene-vinylacetate copolymer (EVA) or a combination thereof. More preferably, thepolyethylene is an ethylene alpha olefin plastomer copolymer(plastomer). The outer layer may also include other thermoplasticmaterials as for example polypropylene, ethylene-propylene copolymer,ionomer, or an alpha olefin which includes members of the polyethylenefamily such as linear low density polyethylene (LLDPE), very low andultra low density polyethylene (VLDPE and ULDPE) respectively, or blendsof these materials.

The inner layer may be made of any suitable thermoplastic polymer thatprovides good heat sealing over a broad range. It is preferred that theinner layer is heat sealable to itself.

By “heat sealable” it is meant that the layer is capable of fusionbonding by conventional indirect heating means that generates sufficientheat on at least one film contact surface for conduction to thecontiguous film contact surface and for the formation of a bondinterface between without loss of the film integrity. Also, the bondinterface may be sufficiently thermally stable to prevent gas or liquidleakage when exposed to above or below ambient temperatures duringprocessing of food within the tube when sealed at both ends, i.e., inbag form. Finally, the bond interface between contiguous inner layersmay have sufficient physical strength to withstand the tension resultingfrom stretching or shrinking due to the food body sealed within thetube.

The inner layer may be made of a heat sealable thermoplastic polymer orpolymer blend. The heat sealable inner layer may be of any materialconventionally used for its heat sealing capability. Heat sealablethermoplastic polymers are recognized by those skilled in the art asbeing capable of heat sealing to themselves at a variety of time,pressure and temperature conditions. For example, at a given pressureeither a relatively high temperature may be applied briefly or a lowertemperature may be applied for a longer period of time to obtainsimilarly suitable seals. It is to be appreciated that persons skilledin the art will be able to select sealing parameters such astemperature, pressure and time of application that depend upon suchfactors as the type of heat sealing equipment used.

The inner layer may be a blend of VLDPE, EVA and a plastomer. Suitably,the blend has combined melting point of about 94-96° C. The EVA may beat least 33% of the blend up to 60%, the VLDPE may be at least 20% ofthe blend up to 33% and the plastomer may be at least 15% of the blendup to 30%. The EVA may contain between about 3% and about 18% vinylacetate.

Other suitable polymers and copolymers may include alpha olefins such asmembers of the polyethylene family such as linear low-densitypolyethylene (LLDPE); very low-density polyethylene sometimes referredto as ultra low-density polyethylene (VLDPE and ULDPE respectively), orblends thereof. Still other suitable thermoplastic polymers as the heatsealable inner layer include polypropylene, ethylene-propylene copolymeror an ionomer. It is to be understood however that the thermoplasticpolymers mentioned herein are not intended to be an exhaustive list, butmerely exemplary.

The inner and outer layers may be adhered together. Alternatively, thefilm may include adhesive, tie or further polymer layers therebetween.Adhesive and tie layer composites includes those which are well known inthe art. Suitably an oxygen barrier layer separates the inner and outerlayers.

In one embodiment, the inner and outer thermoplastic layers are adheredto opposite sides of a barrier core layer. One functional requirement ofthe barrier layer may be that together with the other layers, the oxygentransmission rate through the entire multilayer film may be below about20 cc/1 m²/24 hrs/atm. This is to avoid spoilage of certain foodproducts, for example meat enclosed in the multilayer film package dueto oxygen passage from the environment through the film wall. Thisrequirement may be satisfied by numerous well-known barrier layermaterials. For example, these may include certain of the polyamides(Nylon), hydrolyzed ethylene vinyl acetate copolymer (EVOH) andpreferably a vinylidene copolymer. Vinylidene chloride-vinyl chloride(PVDC) is a commonly used copolymer but vinylidene chloride-methylacrylate copolymer (MA-VDC) may also be used.

The outer thermoplastic layer may be on the opposite side of the corelayer from the inner layer, and in the preferred three layer embodimentthis outer layer is both directly adhered to the core layer and indirect contact with the environment. Since it is seen by theuser/consumer, it may enhance optical properties of the film. Also, itmay withstand contact with sharp objects so it is termed the abuse layerand provides abrasion resistance.

The thermoplastic outer layer may have a thickness of about 0.5 mils to1.0 mils. Thinner layers may be less effective in performing the abuseresistance function, while thicker layers may reduce filmstretchability. The multilayer film may be biaxially oriented to assistin heat shrinkability.

The thickness of the heat sealable inner thermoplastic first layer maybe between about 0.4 mils and about 2.0 mils. Thinner layers may notperform the described functions while thicker layers may not appreciablyimprove processability of the film and may reduce total filmperformance.

The barrier layer thickness may be between about 0.1 mils to about 0.5mils. Thinner barrier layers may not perform the intended functions andthicker layers do not appreciably improve performance.

The three-layer film may have a total thickness of about 1.6 mils toabout 3.0 mils, more preferably from about 1.8 mils to about 3.0 mils.Lower thickness may reduce the effectiveness of at least one of thethree layers while higher thickness may reduce the film flexibility anddo not appreciably improve its performance

In general, various conventional additives such as slip agents andpigments may be incorporated in the film in accordance with conventionalpractice. While this embodiment of the invention is specificallydescribed in terms of three layers, it should be understood that one ormore additional layers may be directly adhered to the outside of theouter layer or between the barrier layer and the inner layer, or inplace of the barrier layer, but not inside the inner layer. Thisadditional layer may for example be EVA, LLDPE, VLDPE, polypropylene,EVOH, polyurethane, acrylonitrile nylon, ionomer, or blends thereof. Forexample, a fourth layer may be interposed between the outer layer andthe barrier layer.

Whilst the advantages conferred from the composition of the outer layermay eliminate the need for irradiation of the multilayer film, it is tobe understood that the multilayer heat shrinkable film in accordancewith the invention may be irradiated to further enhance puncture andtemperature resistance.

The heat shrinkable multilayer film in accordance with the presentinvention may be in the form of a tubular article or flat sheets. Themultilayer film may be formed into bags useful for the packaging ofmeats. The multilayer film may be especially useful for those bagsmanufactured by hot bar or impulse sealing, and after meat is insertedtherein, impulse heat sealing may be used to seal the mouth.

One end of the bag may be heat sealed by adhesion between contiguousinner layer surfaces in a direction transverse to the oppositely locatedsidewalls of the bag. The mouth end of the bag may be impulse heatsealable by fusion bonding between contiguous inner layer surfaces afterfilling the bag with food.

A bag from the multilayer film may be manufactured such that the innerlayer forms the inside portion of the bag while the outer layer formsthe outside portion of the bag. Accordingly, the inner layer of thetubular multilayer film may be the heat sealable layer, which is easilysealed by hot bar sealing. Moreover, because of the latter, the mouth ofthe bag may be more easily sealed by impulse heat sealing.

In a further aspect this invention resides in heat shrinkable multilayerfilm including:

an outer layer of a blend of PB, PP, and either a plastomer or EVA orcombination thereof, whose combined melting point is between about 116°C. to 150° C., and

an inner layer of VLDPE, EVA, and plastomer whose melting point isbetween about 94° C.-96° C.

Suitably, the melting point of the outer layer is between about 118° C.to 137° C.

In order that this invention may be more readily understood and put intopractical effect, reference will now be made to the following exampleswhich illustrate preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A heat shrinkable multilayer film was produced having:

a) an outer layer of a blend of PB, PP, and plastomer whose meltingpoint is between about 129° C. to 136° C.;

b) a barrier core layer

c) an inner layer of VLDPE, EVA, and plastomer whose melting point isabout 95° C.

An alternative heat shrinkable multilayer film was also produced wherebythe outer layer was substituted with an outer layer of a blend of PB,PP, plastomer and EVA whose melting point is about 118° C. A furtherheat shrinkable multilayer film was produced including an outer layercomprising a blend of PB, PP and EVA whose melting point is about 135°C. and a core layer which did not have barrier properties.

In each case, the inner layer included a blend of VLDPE, EVA, andplastomer whose combined melting point is about 95° C. The EVA is atleast 33% of the blend up to 60%. The VLDPE is at 20% of the blend up to33%. The plastomer is at least up to 15% of the blend up to 30%. The EVAmay contain between about 3% and about 18% vinyl acetate.

In general, various conventional additives such as slip agents andpigments were incorporated in the films in accordance with conventionalpractice.

The multilayer film of this invention can be produced by knowntechniques such as by co-extruding the multiple layers into a primarytube, followed by biaxially stretching the tube by known techniques toform a heat shrinkable film. The “double bubble” technique disclosed inPahlke U.S. Pat. No. 3,456,044 can be used to produce the film of thisinvention. Alternatively, the film may be a slot cast co-extrudedmultilayer film, which is subsequently biaxially stretched.

The thickness of the heat sealable inner layer is between about 0.4 milsand about 2.0 mils. The barrier layer thickness is between about 0.1mils and about 0.5 mils. The thermoplastic outer layer thickness isbetween about 0.5 mils and 1.0 mils. Generally, the three-layer film hasa total thickness of from about 1.6 mils to about 3.0 mils, moresuitably from about 1.8 mils to about 3.0 mils.

The bag from the tubular multilayer heat shrinkable film is manufacturedsuch that the inner layer forms the inside portion of the bag while theouter layer forms the outside portion of the bag. Accordingly, the innerlayer of the tubular multilayer film is the heat sealable layer, whichis easily sealed by hot bar sealing. Moreover, because of the latter,the mouth of the bag may be more easily sealed by impulse heat sealingwhen desired.

The multilayer heat shrinkable film may then be employed to manufactureheat shrinkable bags useful in packaging primal and sub-primal meat cutsand processed meats. Bags may be produced from the multilayer heatshrinkable film of this invention by any suitable method, but preferablyby hot bar heat sealing. For instance, if the film of this invention isproduced in a tubular stock, bags can be produced by hot bar sealing oneend of a length of the tubular film or at any number of longitudinallyspaced positions across the tube width, and then cutting the tube orsplitting one edge to form the bag mouth. If the film of this inventionis made in the form of flat sheets, bags can be formed by hot barsealing three edges of two superimposed sheets of film. When carryingout the hot bar sealing operations, the surfaces which are heat sealedto each other to form seams are the first lower melting point outerlayers of the multilayer films of the invention. Thus, for example, whenforming a bag by heat sealing one edge of the length of tubular film,the inner surface of the tube, i.e., the surface which will be heatsealed to itself, will be the inner layer of the tubular multilayerfilm.

EXAMPLE 1

In this series of tests, bags of the same size (400 mm long×200 mm wide)were fabricated from three layer films comprising a heat sealablethermoplastic inner layer, a core layer adhered on one side to thesealable inner layer, and a second thermoplastic outer layer adhered tothe opposite side of the oxygen barrier layer. The core layer in mosttests was a vinylidene chloride copolymer type oxygen barrier. Thethree-layer film was the biaxially oriented heat shrinkable type,prepared by the double or trapped bubble method as broadly described inthe aforementioned Palkhe U.S. Pat. No. 3,456,044. In particular, allthree layers were simultaneously coextruded.

Certain physical properties of bags were measured by either of the testprocedures discussed below.

Shrinkage values were obtained by measuring unrestrained shrink of thestretched film at 90° C. for five seconds. Four test specimens were cutfrom a given sample of the oriented film to be tested. The specimenswere cut to 10 cm in the machine direction by 10 cm. in the transversedirection. Each specimen was completely immersed for 5 seconds in a 90°C. water bath. The distance between the ends of the shrunken specimenwas measured. The difference in the measured distance for the shrunkenspecimen and the original 10 cm was multiplied by ten to obtain thepercent of shrinkage for the specimen. The shrinkage for the fourspecimens was averaged for the MD shrinkage values of the given filmsample, and the shrinkage for the four specimens was averaged for the TDshrinkage value.

The impulse sealing range test is run to determine and compare theacceptable voltage ranges for impulse sealing plastic films. Anevacuator impulse sealer used by the meat industry to evacuate andproducts placed into the bags was used. Boss Vacuum Packaging Machinesmanufacture this evacuator sealer. This evacuator sealer is equippedwith impulse sealing ribbons covered by a Teflon cloth. A constantvoltage is applied to the sealing ribbons and is not adjustable. (Thisis typical for evacuator sealers used commercially by industry.) Thetime that the voltage is applied to the sealing ribbons is adjustable tocontrol the sealing time to obtain a leak proof strong seal. The time isadjustable from 0 to 4.0 seconds or from 0 to 10 on the indicator on themachine.

Film samples are placed in the evacuator sealer and the sealing time isadjusted to determine the minimum time that is required to obtain astrong seal and the maximum time that a good seal can be obtainedwithout burn through.

The results of these tests are summarized in Table A. Table B definesthe formulations of the inner and outer layers. Table C defines theindividual resins that were used. The melting point of the variousblends is calculated by averaging the melting points of thethermoplastic polymers in the blend taking into account the percentageof each polymer in the blend. The percentage of each polymer in theblend is multiplied by its melting point and the sum of these values isthe melting point of the blend.

TABLE A Melt Tem- Shrink Optics Sealing perature ° C. Test MD TD HazeRange Inner Outer No. % % % Gloss Min. Max. Curling Layer Layer 1 48 5011 75 3 3.5 None 95 94 2 45 50 15 70 3 5 None 95 118.5 3 50 52 8.3 83 36 None 95 134.7 4 26 34 11 75 3 6 High 95 135.5 5 37 45 13 62 3 6 High95 131.5 6 45 52 9.5 75 3 5 None 95 129.4 7 44 47 8.5 75 3 5 None 95130.6 8 40 47 12 60 3 7 None 95 136.4 9 45 48 13 75 3 5 None 95 135.0 1032 38 18 65 3 5 None 95 135.0

TABLE B Outer Layer 28% Core Inner Layer 62% Test Type % Type % Type %Type % 10% Type % Type % Type % 1 E1 55 V1 14 P1 28 PVDC E1 40 V1 25 P132 2 E1 40 PB1 20 PP1 21 P1 18 PVDC E1 40 V1 25 P1 32 3 PB1 40 PP1 41 P118 PVDC E1 40 V1 25 P1 32 4 V1 40 PP1 41 P2 18 PVDC E1 40 V1 25 P1 32 5PB1 40 PP2 41 PP3 18 PVDC E1 40 V1 25 P1 32 6 PB1 40 PP2 41 P1 18 PVDCE1 40 V1 25 P1 32 7 PB1 40 PP4 41 P1 18 PVDC E1 40 V1 25 P1 32 8 PB1 41PP5 41 P1 18 PVDC E1 40 V1 25 P1 32 9 PB1 41 PP5 41 P2 18 PVDC E1 40 V125 P1 32 10 PB1 41 PP5 41 E2 18 E2 E1 40 L1 25 P1 32

TABLE C Melt Index g/10 Density Melting Type Description Manufacturermin g/cm³ Point ° C. E1 70% EVA ATO Evatane 0.35 0.935  93 13.5% VAVN1003VN4 13.5% VA 30% EVA VN1005 VN2 0.40 0.928 102 5% VA 5% VA E2 100%EVA VN1005 VN2 0.40 0.928 102 5% VA V1 VLDPE Dow Attane SL 1.00 0.912122 4100 P1 Plastomer Dow Affinity PF 1.6 0.896  94 1140 P2 PlastomerDow Affinity VP 1.0 0.885  74 8770 PB1 Polybutylene Shell 8640 2 118 PP1Polypropylene BASF 1325 8 158 PP2 Polypropylene Montell PLZ860 2.0 0.897140-145 PP3 Polypropylene Montell Adsyl 5 5.50 0.89 135 C 37 F PP4Polypropylene ATO APRYLL 148 3020GN23 PP5 Polypropylene Montell HF 6100161 L1 LLDPE Dow Dowlex 5056 1.1 0.919 122

Test 1 consists of a heat shrinkable non-crosslinked film that is atypical construction used in the industry. The melting temperature ofthe inner and outer layer is the same. While the seal strength of thisfilm meets the requirements for the products the seal range is verynarrow as shown in table A. Typically this type of film is cross linkedby radiation to obtain a maximum setting of about 8.

Tests 2, & 3 use the same inner sealing layer as in test 1 but the outerlayer formulation was changed to use higher melting point resins.Without cross-linking the sealing range was significantly increased.Surprisingly none of the films had curling.

Test 4 and 5 is the same as tests 2 & 3 but the outer layer constructionwas made with different resins. While the sealing range was increasedsignificantly the film curl was not acceptable and the film could not befabricated into bags. The composition of the outer layer did not havecompatible shrink to the inner.

Tests 6, 7, 8 and 9 illustrate the use of different resins andformulations in the outer layers in accordance with the invention. Thesealing range was increased and the films have high shrink properties.

In Test 10 the barrier layer was substituted a non-barrier typematerial. The outer layer is in accordance with the invention. The filmshad a good heat sealing range and shrinkage.

EXAMPLE 2

In addition to the above-described laboratory tests trials wereconducted on a VC999 evacuator, sealer. Bags fabricated from theformulation defined in test 1 were compared to bags fabricated from theformulation in test 3. About 500 bags from each formulation werepackaged in a processed meat product, placed in the VC999 and evacuatedand sealed. The bags containing the product were then immersed in 87Cwater to tightly shrink the bags around the product.

About 15% of the bags made from the test 1 formulation failed at theVC999 seal either after the evacuation, sealing cycle or after shrinkingin the hot water. Only 1.2% failures occurred with bags made from thetest 3 formulation. This failure rate was considered normal for the typeof product that was being packaged and is equivalent to bags made fromfilms that had been cross-linked.

In use, the present invention provides a multilayer film having high hotwater seal strength, high burn-through resistance, a broad impulsesealing range and enhanced puncture resistance without the added step tocross-link the film with irradiation. The multilayer film structure canbe more easily and efficiently formulated into a packaging bag. It alsoprovides a meat and poultry packaging bag having both improved bottomand edge seals formed by e.g. hot bar sealing and after food is packagedtherein, and improved mouth seals formed by impulse heat sealing.

It will of course be realised that while the foregoing has been given byway of illustrative example of this invention, all such and othermodifications and variations thereto as would be apparent to personsskilled In the art are deemed to fall within the broad scope and ambitof this invention as is herein set forth.

What is claimed is:
 1. A multilayer heat shrinkable film including atleast: an outer layer of a thermoplastic copolymer blend including atleast one ethylene alpha olefin plastomer copolymer or at least oneethylene vinyl acetate copolymer, and a heat sealable inner layer of athermoplastic polymer or a thermoplastic polymer blend, wherein saidouter layer has a melting point at least 20° C. higher than said innerlayer and said layers being selected to have substantially balancedshrinkage, and wherein said film is oriented to an extent selected toprovide shrinkability of at least 35% in at least one of machine andtransverse directions, determined by measuring unrestrained shrink ofthe stretched film at 90° C. for five seconds, or equivalent shrinkagethereof.
 2. A multilayer heat shrinkable film according to claim 1,wherein the film shrinks at least 40% in at least one of machine andtransverse directions.
 3. A multilayer heat shrinkable film according toclaim 2, wherein the film shrinks at least 50% in at least one ofmachine and transverse directions.
 4. A multilayer heat shrinkable filmaccording to claim 1, wherein said outer layer has a melting point atleast 30° C. higher than said inner layer.
 5. A multilayer heatshrinkable film according to claim 1, wherein the outer layer blendincludes polypropylene and polybutylene.
 6. A multilayer heat shrinkablefilm according to claim 1, wherein the outer layer blend includespolypropylene, polybutylene, ethylene vinyl acetate copolymer andethylene alpha olefin plastomer copolymer.
 7. A multilayer heatshrinkable film according to claim 5, wherein the melting point of theouter layer blend is about 116° C. to about 150° C.
 8. A multilayer heatshrinkable film according to claim 7, wherein the melting point of theouter layer blend is about 116° C. to about 137° C.
 9. A multilayer heatshrinkable film according to claim 1, wherein the inner layer includesat least one of linear low density polyethylene, very low densitypolyethylene, ethylene vinyl acetate copolymer, ethylene alpha olefinplastomer copolymer or a blend thereof.
 10. A multilayer heat shrinkablefilm according to claim 1, wherein the inner layer is a blend ofethylene vinyl acetate copolymer, ethylene alpha olefin plastomercopolymer and very low density polyethylene or linear low densitypolyethylene.
 11. A multilayer heat shrinkable film according to claim10, wherein the inner layer includes a blend of two ethylene vinylacetate copolymers.
 12. A multilayer heat shrinkable film according toclaim 10, wherein the inner layer includes a blend of 33% to 60%ethylene vinyl acetate copolymer, 15% to 30% ethylene alpha olefinplastomer copolymer and 20% to 33% very low density polyethylene orlinear low density polyethylene based on weight.
 13. A multilayer heatshrinkable film according to claim 10, wherein the melting point of theinner layer blend is about 94 to about 96° C.
 14. A multilayer heatshrinkable film according to claim 1, having at least one thermoplasticpolymer oxygen barrier layer between the inner layer and the outerlayer.
 15. A multilayer heat shrinkable film according to claim 14,wherein the oxygen barrier layer is hydrolyzed ethylene vinyl acetatecopolymer or vinylidene copolymer.
 16. A multilayer heat shrinkable filmaccording to claim 15, wherein the vinylidene copolymer is selected fromvinylidene chloride-vinyl chloride copolymer or vinylidenechloride-methyl acrylate copolymer.
 17. A multilayer heat shrinkablefilm according to claim 1, having at least one thermoplastic polymerlayer between the inner layer and the outer layer, said thermoplasticpolymer layer including at least one of very low density polyethylene,linear low density polyethylene, ethylene vinyl acetate copolymer,ethylene alpha olefin plastomer copolymer or a blend thereof.
 18. Amultilayer heat shrinkable film according to claim 14, wherein thethickness of the outer layer is about 0.5 mils to about 1.0 mils, thethickness of heat sealable inner layer is about 0.4 mils to about 2.0mils, the thickness of the barrier layer is about 0.1 mils to about 0.5mils and the total thickness of the film is about 1.6 mils to about 3.0mils.
 19. A multilayer heat shrinkable film according to claim 1,wherein the film is irradiated.
 20. A multilayer heat shrinkable filmaccording to claim 1, wherein the film is formed into bags.
 21. Amultilayer heat shrinkable film according to claim 28, having at leastone thermoplastic polymer oxygen barrier layer between the inner layerand the outer layer.
 22. A multilayer heat shrinkable film according toclaim 21, wherein the barrier layer is hydrolyzed ethylene vinyl acetatecopolymer or vinylidene copolymer.
 23. A multilayer heat shrinkable filmaccording to claim 22, wherein the vinylidene copolymer is selected fromvinylidene chloride-vinyl chloride copolymer or a vinylidenechloride-methyl acrylate copolymer.
 24. A multilayer heat shrinkablefilm including: an outer layer of a blend including polybutylene,polypropylene and at least one of ethylene alpha olefin plastomercopolymer or ethylene vinyl acetate copolymer, wherein the melting pointof the outer layer blend is about 116° C. to about 150° C., and an innerlayer of a blend including ethylene vinyl acetate copolymer, ethylenealpha olefin plastomer copolymer and very low density polyethylene orlinear low density polyethylene, wherein the melting point of the innerlayer blend is about 94° C. to about 96° C.
 25. A multilayer heatshrinkable film according to claim 24, wherein said film shrinks atleast 35% in at least one of machine and transverse directions bymeasuring unrestrained shrink of the stretched film at 90° C. for fiveseconds, or equivalent shrinkage thereof.
 26. A multilayer heatshrinkable film according to claim 24, wherein the melting point of theouter layer blend is about 117° C. to about 137° C.
 27. A multilayerheat shrinkable film according to claim 24, wherein the outer layer is ablend of polypropylene, polybutylene, ethylene vinyl acetate copolymerand ethylene alpha olefin plastomer copolymer.
 28. A multilayer heatshrinkable film according to claim 24, wherein the inner layer blendincludes 33% to 60% ethylene vinyl acetate copolymer, 15% to 30%ethylene alpha olefin plastomer copolymer, 20% to 33% very low densitypolyethylene or linear low density polyethylene based on weight.
 29. Amultilayer heat shrinkable film according to claim 24, having at leastone thermoplastic polymer layer between the inner layer and the outerlayer, said thermoplastic polymer layer including at least one of verylow density polyethylene, linear low density polyethylene, ethylenevinyl acetate copolymer, ethylene alpha olefin plastomer copolymer or ablend thereof.
 30. A multilayer heat shrinkable film according to claim24, wherein the thickness of the outer layer is about 0.5 mils to about1.0 mils, the thickness of heat sealable inner layer is about 0.4 milsto about 2.0 mils, the thickness of the barrier layer is about 0.1 milsto about 0.5 mils and the total thickness of the film is about 1.6 milsto about 3.0 mils.
 31. A multilayer heat shrinkable film according toclaim 24, wherein the film is irradiated.
 32. A multilayer heatshrinkable film according to claim 24, wherein the film is formed intobags.